1. Field of the Invention
This invention relates to a vehicle slip control system for restraining an excessive slip of driving wheels of the vehicle which occurs during starting acceleration of the vehicle, for example.
2. Description of the Related Art
In JP-A-62-121839, for example, there are shown types of systems for controlling a position of a blade of throttle valve so that the slip ratio S=(Va-Vb)/Va, determined by a vehicle speed Vb obtained from driving wheels and a driving wheel speed Va, stays within a predetermined range in the event of a slip of the driving wheels.
There are also known techniques for restraining a wheel slip by limiting an engine torque by stopping a fuel supply to an internal combustion engine on the vehicle or by delaying the ignition timing (JP-A-63-263243 and U.S. Pat. No. 4,721,176). Furthermore, there have been proposed techniques for adjusting the throttle position, for example, according to a steering angle or the like to prevent slippage during turning (JP-A-63-18144, JP-A-63-149236, and JP-A-63-32136).
In the arrangements disclosed in the above-mentioned patent publications, it is often impossible to provide a suitable control over the engine torque to be controlled in the event of slippage because the engine torque differs with different road conditions (e.g., snowy roads, icy roads) expressed by a friction coefficient .mu. between the road surface and the driving wheels and also differs with the kind of the tire.
In the so-called traction control to prevent a wheel slip, the optimum torque restraint rate (by throttle control, for example) at the time of slippage differs with the friction coefficient of the road surface, and therefore, in order to exert control with an adequate torque regardless of the road condition, it is necessary to know the friction coefficient .mu. and perform traction control according to this friction coefficient. In the prior arts, however, an adequate friction coefficient .mu. cannot be estimated.
To take an example, in a technique disclosed in JP-A-2-19622, the friction coefficient .mu. of the road surface is estimated from the acceleration of the driving wheels in the event of slippage. However, in this technique, since the drive wheel torque is deemed constant, it sometimes occurs that the variations of the actual drive wheel torque are reflected in the above estimated friction coefficient .mu. of road surface, making it impossible to estimate a correct friction coefficient .mu..
JP-A-1-122763 discloses a method for estimating the friction coefficient of a road surface which takes the drive torque of the driving wheels into account, but this method does not consider a road pressing force of the driving wheels, so that it is difficult to estimate an accurate friction coefficient of the road surface.